This disclosure is related generally to the field of marine surveying. Marine surveying may include, for example, seismic and/or electromagnetic surveying, among others. For example, this disclosure may have applications in marine surveying in which one or more geophysical sources are used to generate energy (e.g., wavefields, pulses, signals), and geophysical sensors—either towed or ocean bottom—receive energy generated by the geophysical sources and possibly affected by interaction with subsurface formations. Geophysical sensors may be towed on sensor cables (referred to as streamers). Some marine surveys locate geophysical sensors on ocean bottom cables or nodes in addition to, or instead of, streamers. The geophysical sensors thereby collect geophysical survey data (or simply “survey data”) which may be useful in the discovery and/or extraction of hydrocarbons from subsurface formations.
In marine surveying, a streamer is typically a cable assembly that can be stored on a drum on the towing vessel. The streamers are typically made of multiple components, such as electrical conductors, fiber optics, and stress-supporting members, all bundled together and covered with a protective outer skin. The streamer may be up to several kilometers in length. In general, the streamer has little stiffness in directions other than inline, so it can move easily both laterally and in torsion/rotation when deployed in the water. When sensors such as velocity, position, acceleration, and/or electromagnetic sensors are incorporated into the streamer, the streamer motion, rotation, and/or movement through a background magnetic field may be sensed directly by the sensors. This may produce a level of noise which may degrade the data acquired during the marine survey. For example, the noise may be measurements of local conditions in the surrounding water (e.g., currents) rather than geophysical signals from the subsurface formation.
Electromagnetic surveying may include measurements of electric fields, magnetic fields, or a combination of the two. Such measurements may measure specific components of the field(s), such as in-line or cross-line, or measurements may obtain data indicative of the absolute value or vector direction of the total field(s). For example, magnetometers for use in electromagnetic surveying may be divided into two principal classes:                Vector magnetometers that are sensitive in a certain direction. These may be grouped to provide simultaneous measurements in two or three orthogonal directions.        Total field magnetometers that are sensitive to the total field, but do not map the magnetic field vectors.Vector magnetometers may provide directional information, but may not accurately map the total magnetic field while moving—motion noise may be translated into magnetic field noise. This reduces the benefits of using conventional vector magnetometers in a towed electromagnetic sensor cable.        
Total field magnetometers (“TFMs”) may monitor changes of the total magnetic field at positions on a moving platform. TFMs may be subject to less motion noise than vector magnetometers. TFMs have been used in airborne applications. However, conventional TFMs are large (e.g., a sensor head larger than 100 cm3), expensive, and excessively power consuming (e.g., greater than 20 Watts). One example marine magnetometer shown in FIG. 1 weighs between 40 lb and 50 lb, and is between 4 ft and 5 ft in length. Moreover, conventional cesium magnetometers may have a “blind spot” around the poles of the sensor head resulting in reduced sensitivity to fields in the direction of the poles.
In some electromagnetic surveys, the background magnetic field is assumed to not vary or to smoothly vary across the streamer spread. For example, the Earth's magnetic field may be modeled as varying linearly with distance across the streamer spread. Survey data may then be adjusted by (1) measuring the background magnetic field at several points, (2) modeling the background magnetic field across the entire streamer spread, and (3) subtracting the modeled background magnetic field from the acquired electromagnetic data. However, the linear assumption may fail for electromagnetic surveys near the Earth's poles.
Total magnetic field measurements on geophysical streamers would benefit from new equipment and/or methods that provide better motion noise reduction or mitigation. Survey data quality would benefit from better measurements of the background magnetic field.